Squeezed state light field is an important quantum resource in the aspect of gravitational wave detection, optical precise measurement, quantum information, etc. In the experiment preparation system of the squeezed state light field, the performance of the servo-control system is the key technique to obtain the squeezed state light field with high performance. However, in the feedback control system based on the electro-optic phase modulation, the introduction of the residual amplitude modulation during the process of the electro-optic phase modulation, so that the zero baseline of error signals of PDH locked by sideband frequencies drifts with time to influence the performance of the feedback control system, is inevitable. The frequency detuning of the OPO (optical parametric oscillator) cavity will cause compression angle rotation, and the drift of phase lock point will cause the anti-compression component to mix into the compression component, which is adverse for lowering the noise of quantum.
In the PDH locking system, the electro-optic phase modulator is widely used as an optical modulator. Due to the electro-optic effect of electro-optic crystal, the distribution of the refractive index in each direction of the electro-optic crystal will change, the phase of the optical wave can be modulated by the electro-optic phase modulator. The characteristics of equal amplitude and opposite phase of the two modulated side bands are used to obtain the spectral line of the frequency discrimination signal, so as to lock the optical cavity length and the relative phase in the central zero crossing point to realize the stability of the optical cavity length and the relative phase. It is found from the experiment that after the laser is electro-optic phase modulated, the phase modulation and the residual amplitude modulation (RAM) are generated at the same time, showing that the positive and negative first side bands of the modulated light are not of exactly equal amplitude and opposite phase. Moreover, the asymmetry of the side band will change when the environment and experimental conditions change. In the locking system, theoretical calculation and experimental results show that the existence of the residual amplitude modulation will cause the central zero point of the spectral line as the frequency discrimination signal to shift, so that the accuracy of controlling and locking is affected, leading to certain errors of the optical cavity and the lock point of the relative phase due to the existence of the residual amplitude modulation. The drift of locked loop is increased under the influence of the residual amplitude modulation, which is adverse for the stable running of the system. Besides, in the precise control of the light field, theoretical calculation and experimental results show that the existence of the residual amplitude modulation will cause the central zero point of the spectral line of the frequency discrimination signal to shift, so as to affect the accuracy of measuring and controlling. In the precision laser spectrum, the fitting of the spectral line of the molecular light spectrum will generate a certain error because of the existence of the residual amplitude modulation. The drift of frequency locked loop is increased under the influence of the residual amplitude modulation, which is adverse for the stable running of the system. With the improvement of the experimental conditions and the further improvement of the experimental requirements, the influence caused by the residual amplitude modulation has been paid more and more attention.
The main reasons causing the residual amplitude modulation are the birefringence effect, the etalon effect, the non-uniform distribution of the electric field, the radio-frequency power jitter, the laser frequency jitter, etc. The present invention mainly focuses on theoretical and experimental research on reducing the residual amplitude modulation in phase modulation and mainly studies the influence of the electric field distribution of the electro-optic modulated crystal on the residual amplitude modulation. The influence range of the edge effect of the electro-optic modulated crystal is calculated and the volume of the uniform electric field of the electro-optic modulated crystal is enlarged. A device for reducing residual amplitude modulation is provided, which includes an electro-optic modulation crystal, a high-frequency signal source module. The laser passed through the combined modulated crystal, which is connected to the high-frequency signal source. The high-frequency signal source transmits a sine wave signal, causing the phase modulated laser to generate the modulated light. Therefore, the laser passes through the uniform electric field of the electro-optic modulated to reduce the residual amplitude modulation, so as to improve the performance of the feedback control system, which is beneficial for obtaining a squeezed state light field with high compression degree and stable operation.
The research in the prior art shows that the main reasons causing the residual amplitude modulation are the birefringence effect and the etalon effect of the crystal. Most of the prior arts suppress the residual amplitude modulation by altering the voltage applied to the electro-optic crystal so as to change the distribution internal electric field of crystal. Actually, the crystal itself affects the internal electric field. However, the influence of the electric field distribution of the electro-optic crystal itself on the residual amplitude modulation has not been considered before.